1. Field of Art
This invention relates to improvements in or relating to mass flow measurement, which is the measurement of the mass flow rate of so-called "bulk" materials. Such materials include but are not limited to grain, forage, granular chemicals, powders, fruit, vegetables, coal, minerals, ores and high viscosity liquids. The invention has particular application to the measurement of the mass flow rate of grain to the grain tank of a combine harvester.
2. Background of Prior Art
It is known to provide a mass flow measuring apparatus including a sensor member having a surface along which the streamline flow of bulk material may occur thereby giving rise to forces, moments or torques in the sensor member that are measurable. Strain gauges, optical displacement sensors, ring dynamometers and torque meters can be used to produce electrical signals indicative of the magnitudes of the forces, moments or torques, thereby providing an indication of the mass flow rate of bulk material.
Co-pending European Patent Application No. 96201889.1 discloses a method and apparatus for rendering the measurements of mass flow rate measuring apparatuses more accurate, by substantially eliminating the effects of changes in the coefficient of friction (.mu.) between the bulk material and the sensor surface on the measured mass flow rate values. Nonetheless, there is scope for improvement of the accuracy of such measuring apparatuses, as detailed below.
Furthermore, there are additional kinds of mass flow rate measuring apparatuses that rely on the establishment of impulse forces on a sensor plate, instead of the establishment of streamline flow. Such impulse type measuring apparatuses are believed to be less accurate than the streamline flow type. Thus there is a greater need for improving the accuracy of such apparatuses.
Mass flow measuring apparatuses may be employed in grain and chemical hoppers, railway tankers and waggons, silos and crop harvesting machines such as combine harvesters.
In combine harvesters it is highly desirable to know the rate at which crop grains are filling the grain tank of the harvester during harvesting operations. It has been proposed to use the streamline flow type mass flow measuring apparatuses in combine harvesters. It is essential in such apparatuses to know the velocity of grain at the start of its travel along the sensor surface.
It has been proposed to locate the sensor surface at the top of the grain elevator leading from the threshing mechanism of the combine harvester to the base of the so-called "bubble up" auger that feeds grain into the grain tank. The grain elevator is a paddle-type device having a series of shaped paddles or buckets mounted on an endless drive chain or other flexible member that is powered at a constant speed from the engine of the vehicle. The flexible chain extends substantially vertically over the majority of its length, and is positioned such that the paddles or buckets repeatedly pick up quantities of grain at a low level in the cleaning area below the threshing area of the combine harvester and raise them to the bubble up auger. The motion of the grain elevator is such as to throw the quantities of grain clear of the paddles, towards the bubble up auger, when they reach the correct height.
Previous proposals for mass flow measuring apparatuses involve locating a sensor surface so that it forms part of the path of the grains between the paddles at the top of the elevator and the base of the bubble up auger. Thus it is possible to establish streamline flow of grains on the sensor surface by guiding the projected grains (via a further surface at a tangent to the end of the sensor surface) to the end of the sensor surface.
Generally, the sensor surface lies above the grains for the greater part of their travel therealong, the forces, moments or torques in the sensor member being generated principally by centrifugal forces transmitted from the grains to the surface: and by the effects of friction between the gains and the material of the sensor surface.
Since each paddle or bucket on the grain elevator contains grain to a depth of several centimeters, it will be appreciated that not every grain from a paddle or bucket on the elevator is projected at the same velocity and with the same momentum. Thus some of the grains fail to reach the sensor member and tend to fall downwardly beside the grain elevator. Such grains may fall to the bottom of the conveyor housing and add to the volume of freshly threshed grain. Eventually, recurrently recycled grains become milled between the moving components and the walls of the conveyor.
The grains that fall away from the sensor member can lead to inaccuracies in the measured flow rate.